A high speed photographic printer typically includes a light source under which a roll of developed film containing negative images is rapidly and continuously passed for reproducing the images on a roll of photosensitive paper, which is placed beneath the film. A lens assembly is placed between the paper and the roll of film for focusing the image and the amount of light which is directed onto the paper. The lens assembly includes two moveable groups of elements for controlling the magnification of the image and an iris diaphragm for controlling the amount of light.
The typical iris diaphragm includes a plurality of moveable blades for forming an adjustable aperture through which the light passes. The blades are typically controlled by a stepper motor which, via movement of the blades, induces the variation of the aperture. This configuration is disclosed in detail in U.S. Pat. No. 4,444,489.
Although the above described system and method for varying the aperture are satisfactory, they are not without drawbacks. It takes approximately two or three seconds to adjust the diameter of the aperture, and for high speed photographic printers, such a response time is insufficient to allow the aperture to change between images on the same roll of film because of the rapid speed at which the film is moving under the light source. Therefore, the entire roll of film is printed at one aperture setting, and if changes are needed for any of these printed images, the aperture is then adjusted and the copying process repeated. This consumes time, which obviously adds additional cost to the printing process.
In addition, the stepper motor and its associated blades should be replaced after approximately two million actuations because experience has shown this to be its expected lifetime. For high speed photographic printers, two million actuations are consumed in a short period of time. For this reason, the durability of the stepper motor arrangement is inadequate for high speed photographic printers.
Finally, the stepper motor arrangement consumes a great amount of space. This is obviously undesirable because it increases the size of the photographic printer.
To meet the above stated stringent performance requirements, it is necessary to employ a variety of unusual materials. These materials are chosen for their mechanical and physical properties; however, assembly of the individual parts is a problem. Common fastening methods such as threaded fasteners, ultrasonic welding, or adhesive bonding are not possible for the following reasons. First, material incompatibility between various parts prevents adhesive bonding and effective ultrasonic welding. The desire to minimize inertia and the fact that some components are physically small precludes the use of common threaded fasteners. Also, the required precision eliminates the use of any rivet type assembly technique. Finally, precision kinematic requirements demand an assembly process where all pre-stresses are controlled or minimized.
Consequently, a need exists for improvements in the construction of iris diaphragms so as to meet the stringent requirements for high speed photographic printers, and so as to overcome the above-stated inherent drawbacks of mechanical bonding associated with such a construction.